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AR# 9819

Description

What are the I/O standard, drive strength, and slew rate for the dedicated configuration and JTAG pins?

解决方案

All of these pins, in all device families except Virtex-II Pro and Spartan-3, are configured as LVTTL with 12 mA drive and fast slew rate. One exception is CCLK on Virtex-II devices, which is configured with a slow slew rate (LVTTL12S).

For Virtex-4 / Virtex-5, the dedicated configuration pins are LVCMOS 12 mA Fast Slew, where the voltage is determined by Vcc_Config.

NOTES:

1. The DONE pin (all families) is an open-drain driver by default. In this case, the output behavior is dictated by the pull-up resistor attached to it.

2. Not all configuration/JTAG pins are fully dedicated. If they are used as user I/O subsequent to configuration (e.g., /INIT can be configured as user I/O), you can determine the drive characteristics with the IOSTANDARD attribute (LVTTL12F by default, LVCMOS25 for Spartan-3 and Virtex-II Pro, by default). Refer to the "Pin Description" section of the appropriate device data sheet to determine which pins are fully dedicated and which pins can be used as user I/O.

3. To predict I/O behavior, an IBIS simulation is always necessary.

Use special care when creating a JTAG chain that contains a Spartan-3 family part with 3.3V parts. To allow Spartan-3 JTAG pins to interface with 3.3V signals, you must use current limiting resistors.